The present invention relates to a method and apparatus for data transmission by a MS in a Time Division Multiple Access (TDMA) system. More specifically, the present invention relates to providing power information concerning the mobile station (MS) to the mobile network to implement multi-slot transmission of data services in such a system.
TDMA is a fundamental digital telecommunication technology and forms the basis of cellular standards such as Global System for Mobile Communications (GSM). A TDMA system includes the basic subsystems of cellular radio systems including a switching network, base stations (BSS""s) and MSs. In a TDMA system the radio spectrum is divided into radio carrier frequencies typically spaced 30 kHz to 200 kHz apart. Digital techniques are employed at the BSS and in the cellular radio to subdivide the time on each radio channel into time slots, i.e. the TDMA radio carrier waveform is divided into several different types of control and voice/data channels by the use of different time slots or shared portions of time slots.
Time slots are the smallest individual time periods available to each MS. Each time slot can be assigned to a different MS, and the time slots can be dedicated or dynamically assigned. Voice or data information as well as access information are converted to digital information that is sent and received in bursts during the time slots. The entire repeating pattern of the time slots is called a frame, which includes eight time slots. Thus, TDMA systems allow several MS""s to operate simultaneously on a single radio carrier frequency because the MS""s share the radio frequency by dividing their signals into slots.
Data services ordinarily are communicated in the same way as voice signals, that is they are assigned a particular individual time slot in a frame and share a single bandwidth radio carrier with other users. However, whilst the transfer of voice signals must be on a real time basis (instantaneous), certain data services can be transferred on a non-real time basis (stored or delayed).
Non-real time operability of data services allows transmission of the information to be treated with increased flexibility. This has led recently to the development of multi-slot data transfer capability. Multi-slot transmission is facilitated by the network, typically the BSS, which controls channel allocation to users.
Communication to effect data services can also be implemented in a packet switched network which could for example operate according to the General Packet Radio Services (GPRS) standard or High Speed Circuit Switched Data (HSCSD). GPRS uses a packet mode technique to transfer high-speed and low speed data using packets in one or more time slots per frame. Communication in a packet switched network operated is implemented using two network nodes, a Serving Support Node and a Gateway Support Node. These nodes correspond to a Serving GPRS Support Node (SGSN) and a Gateway GPRS Support Node (GGSN) in a GPRS network.
The SGSN of the GPRS network keeps track of the MS""s locations and performs security functions and access control and is connected to the BSS. The GGSN of the GPRS network provides interworking with external packet switched networks and is connected to the SGSN. HSCSD is based on the use of up to 8 time slots per TDMA frame for one communication, thus providing for higher data transmission speeds. Typical applications of HSCSD include, facsimile, file transfer, electronic mail, video and distribution services. The discussion below shall proceed based on the use of a GPRS network. Similar concepts apply to HSCSD.
In a GPRS network information is provided for the MS to be able to detect predetermined states or conditions of the MS and determine when to perform a Routing Area Update (RAU) procedure. These predetermined states could for example include a state where a new cell or a new Routing Area (RA) has been entered by the MS. The MS detects that a new RA has been entered by comparing the Routing Area Identity (RAI) stored in the MS with that received from the new cell.
Other predetermined states that may cause a RAU procedure to be conducted could for example be when the periodic RA update timer has expired, when a suspended MS is not resumed by the BSS, etc.
When a RAU request is issued by the MS, the SGSN detects whether the RAU request is an intra SGSN RAU or an inter SGSN RAU by determining whether it also handles the old RA. In the case of an intra SGSN RAU, the SGSN has the necessary information about the MS and there is no need to inform the GGSNs or the HLR about the new MS location. A RA update is always an intra SGSN routing area update.
A normal Intra SGSN RAU procedure as illustrated in FIG. 1 includes the following. The MS sends a RAU Request (old RAI, old P-TMSI Signature, Update Type) to the SGSN (Step 1). The Request indicates whether it is a normal RAU, periodic RAU or combined LA/RA update. The BSS adds the Cell Global Identity including the Radio Access Capability (RAC) of the cell where the message was received before passing the message to the SGSN. Security functions are executed if implemented in the network (Step 2). The SGSN validates the presence of the MS in the new RA (Step 3). If due to regional subscription restrictions the MS is not allowed to be attached in the RA, or if subscription checking fails, then the SGSN rejects the RAU with an appropriate cause. If all checks are successful then the SGSN updates the Mobility Management (MM) context for the MS. A RAU Accept (P-TMSI, P-TMSI Signature) is returned to the MS. If P-TMSI was reallocated, the MS acknowledges the new P-TMSI by returning a RAU Complete message to the SGSN (Step 4). If the RAU procedure fails a maximum allowable number of times, or if the SGSN returns a RAU Reject (Cause) message, the MS shall enter IDLE state.
A normal Inter SGSN RAU procedure is similar to the normal Intra SGSN RAU procedure except that communications are performed between the new SGSN and the old SGSN and the GGSN and the Home Location Register (HLR) so as to transfer and update context and location information regarding the MS in the new SSGN, the GGSN and the HLR.
Current design of the transmitter of a MS does not allow it to satisfactorily cope with multi-slot transmission. One particular drawback is that the output RF power amplifier in standard TDMA MSs are designed to transmit at a rate of one slot per frame. In multi-slot transmission, driving the power amplifier at increased rates introduces new design challenges. Specifically, the power amplifier of the transmitter accordingly is driven at an increased rate compared to normal transmission at one slot per frame. These longer transmission periods may cause higher Specific Absorption Rate (SAR) values and in rare instances the measured SAR values might exceed predetermined limits. SAR values assess levels of human exposure to radio signals from wireless communications devices deemed to be appropriate by various standards organizations.
In multi-slot transmission operation, the transmitter outputs on more than one time-slot per TDMA frame. Thus, the power amplifier of the transmitter is driven at an increased rate compared to normal transmission at one slot per frame. This leads to an increase in the operating temperature of the power amplifier, which can introduce problems due to the thermal effects. For example, if the junction temperature of the RF power amplifier exceeds its upper rating, then changes could occur in the amplifier""s operating characteristics. If multi-slot transmission is sustained the temperature of the power amplifier will continue to rise and if left unchecked, the rise in operating temperature of the power amplifier could possibly lead to irreversible damage.
The rise in temperature of the power amplifier could also negatively affect other elements of the MS, such as the battery, display, IC chips, housing, etc. For example, continuous heating of the battery could damage the battery or cause the battery to drain faster than normal. Therefore, the transmission power of the MS is monitored so as to prevent the MS from exceeding its maximum allowed transmission power. Accordingly, a solution to this problem is important.
One approach to solving this problem could be to increase the power amplifier capacity and introduce further heat sinks in the MS. However, this would necessitate an increase in the size of the MS, which would be undesirable given the general trend towards more compact MSs. Another approach to solving this problem is to limit the power output of the power amplifier while continuing to transmit in the multi-slot mode by controlling the maximum allowed output power levels.
The maximum allowed power output of a MS transmitter depends on its allocated power classification, which is network controlled. The network chooses the transmission power of the MS, and commands to regulate it are issued to the MS. The network determines the required MS transmission power through reception level measurements performed at the BSS, taking into account the MS maximum transmission power as well as quality measurements done by the network. This last parameter helps to ensure that transmission quality is kept above some acceptable threshold. This information is used for power control and handover preparation.
GSM has five power classmarks defining respective maximum output powers. Typical classes in GSM900 are class 2 for transportable mobile telephones (e.g. vehicle-mounted equipment) with a maximum power output of 8 watts; hand portables are classified 3 through to 5 with maximum power outputs ranging from 5 to 0.8 watts.
Whilst the classmark is sent by the MS in the initial message at the beginning of the data transmission, it may happen that the classmark changes during the transmission. To implement this change a classmark change procedure must be executed. In the GSM specification the classmark change procedure is described in GSM 04.08 in section 3.4.10. For a packet switched MS which operates for example according to the GPRS standard in a GPRS Network, multi-slot communication can be conducted with the GPRS network (i.e., GPRS attached). However, it may happen that the transmission power class of the GPRS MS need to be changed and indicated according to a user situation when, for example, an external interface (IR, Bluetooth) is used.
In conventional apparatus GPRS communication with the GPRS Network has to be terminated (GPRS Detached) and re-established (GPRS Attached) when the transmission power class change need to be indicated to the network. In conventional GSM apparatus the transmission power class change can be accomplished using the classmark change procedure, which is performed between the MS and MSC. However, the classmark change procedure cannot be utilized in a GPRS Network since nodes SGSN, GGSN are used. In GPRS, the MSC is not involved in GPRS signaling, only the SGSN and GGSN. Further, the RAU procedure does not provide a mechanism to change the transmission power class when a RAU is conducted. Therefore, in a GPRS Network a mechanism for indicating a transmission power class change during the GPRS attached is needed.
The present invention proposes that a MS use the RAU procedure to indicate the transmission power class change to the SSN of a packet switched network. The present invention could, for example, be implemented in a GPRS network wherein the MS uses the RAU procedure to indicate the transmission power class change to the SGSN during the GPRS attached. The SGSN then conveys the transmission power class change to the BSS.
According to the present invention the MS is allowed to indicate a new transmission power class in the MS RAC field of a RAU Request message sent to the SGSN during a RAU procedure. The RAU Request message of the RAU procedure contains a field having Update Type information, which indicates whether the MS is requesting a normal RAU procedure, a periodic RAU procedure or combined RA/LA update procedure. Any of these can be used to indicate a transmission power class change in the MS or new Update Type can be introduced for this purpose. If the RAU Request message provides a new transmission power class of the MS, then the SGSN shall accept the new transmission power class and deliver the new transmission power class information to the BSS.